The present invention relates to the treatment of urea solutions which come from synthesis reactors where ammonia (NH.sub.3) and carbon dioxide (CO.sub.2) are reacted under pressure and at high temperature to form urea, and which contain, in addition to the formed urea, free ammonia, unreacted ammonium carbammate and water together with other by-products, the treatment consisting of:
(a) a first phase or step in which an important amount of NH.sub.3 is distilled from the urea solution while decomposing only a reduced amount of carbammate; and PA1 (b) a second phase in which the major part of the undecomposed carbammate present in the so treated solution is decomposed preferably under the contemporaneous stripping action of fresh CO.sub.2.
The invention concerns also a device for the embodiment of the above treatment.
In the industrial urea production by NH.sub.3 and CO.sub.2 synthesis, the so-called "stripping" technologies have been mostly used. According to such conventional technologies, the residual unreacted carbammate in the urea melt coming from the reactor, is decomposed at high pressure (decomposer or stripper operating at the reactor pressure), because of NH.sub.3 or CO.sub.2 which is present in large excess in the decomposer vapor phase. According to the German Auslegeschrift No. 1468628 (I) and Dutch patent application No. 7019056 (II) such excess is obtained by external supply at the stripper bottom of gaseous NH.sub.3 (reference I) or CO.sub.2 (Reference II), countercurrently with the urea melt.
According to U.S. Pat. No. 3,876,696 (III) the NH.sub.3 excess is ensured by the fact that the urea solution feeding the stripper, is already rich in NH.sub.3 and the carbammate decomposition takes place in an exchanger in which the liquid falls as a thin film.
Finally, according to the Italian Pat. No. 770,241 (IV) carbammate decomposition is achieved, at a pressure considerably lower than that of the synthesis reactor, in two stages in the second stage of which external ammonia is introduced countercurrently. With this system the residual NH.sub.3 content in the treated urea melt is very high (35.7 wt.%) and expensive NH.sub.3 recovery stages are required; pumping devices are also required to recycle to the reactor the carbammate solution available at lower pressure.
In the process and plants operating according to reference patents I, II, III, all vapors produced in the carbammate decomposition, together with the distilled free ammonia, and together with all NH.sub.3 (reference I) or CO.sub.2 (reference II) introduced at the decomposer bottom, flow upwards counter currently with the urea melt, which leaves the decomposer from the bottom.
This method causes high process flows (vapor and liquid flows) through the carbammate decomposer tubes. Sizing of decomposer tubes (tube number and diameter) becomes very critical and selection cannot be optimised in relation to the process performance, because of the limitations imposed by the necessity to avoid flooding phenomena. Such phenomena adversely affect the decomposer performance and might damage it, thus reducing its operating life. Furthermore, large quantities of passivating agent are generally needed to prevent corrosion of the large heated stainless steel surfaces of the above-mentioned falling-film decomposers. Air is generally injected into the decomposers, thus utilizing the passivating action of oxygen. However, the introduction of large amounts of air into the system is detrimental to conversion in the reactor. Inert materials also cause ammonia and CO.sub.2 losses through urea plant units, while oxygen is a potential source of explosions.